If you make all events public, you end up sending all scroll events and mouse move events to the app's top-level signal just because some component needs them. The larger your app is, the more likely it is that some component somewhere will need an event sooner or later.

Traditional UI frameworks try to avoid sending events unnecessarily for performance reasons; they're designed so that each component can declare which events it's listening to and can start and stop listening as needed.

Perhaps Elm can make this work, so that unwanted events get filtered out so quickly that it's not a concern. Also, these days everything gets animated anyway based on things like scroll events, so apps are perhaps a bit more like video games that run at 60 frames/second regardless of what's going on, at least while they're in the foreground. And the browsers themselves perform a lot better.

But it shouldn't be hard to see why people who have experience building UI frameworks might be concerned about unnecessary event traffic, particularly on mobile where battery life is an issue.